Net Earnings Per Share (Schedule of Earnings Per Share, Basic and Diluted) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		6 Months Ended
	Oct. 31, 2016	Oct. 31, 2015	Oct. 31, 2016	Oct. 31, 2015
Earnings Per Share [Abstract]
Net income	$ 17,637	$ 18,180	$ 39,298	$ 33,338
Denominator for basic net earnings per common share - weighted-average shares	16,295,948	16,282,881	16,280,164	16,231,870
Effect of dilutive securities:
Stock options and restricted stock units	144,000	190,000	131,000	215,000
Denominator for diluted net earnings per common share - weighted-average shares and assumed conversions	16,440,321	16,472,645	16,410,652	16,446,937
Net earnings per share
Basic (in usd per share)	$ 1.08	$ 1.12	$ 2.41	$ 2.05
Diluted (in usd per share)	$ 1.07	$ 1.10	$ 2.39	$ 2.03